1. Field of the Invention
The present invention relates to a microwave baking furnace for baking an object to be baked which is made of a pottery material or a fine ceramics material.
2. Description of the Related Art
Recently, a technique in which the pottery material and the fine ceramics are baked by microwave heating is suggested, and this technique has already been put to practical use.
When an object to be baked is baked by the microwave heating, and the object to be baked is homogeneous, the microwave uniformly heats each part of the object to be baked in principle. However, since an atmosphere temperature is considerably lower than a surface temperature of the object to be baked at the beginning of a baking process, heat is radiated from the surface of the object to be baked. As a result, a temperature gradient occurs between a central portion of the object to be baked and the surface thereof and crack easily occurs.
Further, when an object to be baked is made of the same material, as characteristics of the microwave heating, dielectric loss becomes larger as temperature rises up. Therefore, if the temperature gradient occurs, a microwave absorption rate of a high-temperature portion is high, the difference in microwave absorption rate is further progressed, and local heating occurs partially.
When the temperature gradient occurs in this way, the difference in temperature further increases due to the microwave heating. As a result, the occurrence of the crack is assisted.
Further, in the baking using the microwave heating, in case that an object to be baked is made of a material such as alumina or silica, which is a main material of ceramics and has a low dielectric loss at room temperature, there is a problem in that the energy efficiency of microwave heating in a low-temperature zone is low.
Therefore, as the microwave baking furnace for suppressing such a temperature gradient and for reducing the occurrence of the crack, a microwave baking furnace having the structure shown in FIG. 5 is suggested (for example, refer to Japanese Unexamined Patent Application Publication No. 2002-130960 (Page 3, FIG. 1)).
A microwave baking furnace 1 includes a cavity 3 partitioning a microwave space 2, a magnetron 6 as a microwave generating means which is connected to the cavity 3 via a waveguide 4 and radiates microwave to the inside of the cavity 3, a microwave stirring means 8 for stirring the microwave radiated to the inside of the cavity, a blanket 10 arranged inside the cavity 3, and an auxiliary blanket 11 surrounding the blanket 10.
The cavity 3 reflects the microwave toward the microwave space 2 at least at the inside thereof and prevents the microwave from leaking.
The microwave stirring means 8 has stirring blades 14 disposed inside the cavity 3, a driving motor 16 disposed outside the cavity 3, a rotation transmitting shaft 18 for transmitting the rotation of the driving motor 16 to the stirring blades 14. The atmosphere in the cavity 3 is stirred by the rotation of the stirring blades 14.
The blanket 10 partitions a baking chamber 23 in which an object to be baked is disposed. A partition wall 25 partitioning the baking chamber 23 is constructed as a double wall structure of an outer wall 25a and an inner wall 25b. 
The outer wall 25a is made of a material which has insulating properties and permits the microwaves to be transmitted therethrough. Specifically, the outer wall 25a is made of alumina fiber or foamed alumina.
The inner wall 25b is made of a dielectric material which self-heats by the microwave radiated thereto from the outside and which can transmit part of the microwaves to the inside of the baking chamber 23.
As a preferred dielectric material for the inner wall 25b, for example, a heating material for a high-temperature zone, which self-heats equally to or more than an object to be baked in a high-temperature zone near a baking temperature. In case that the object to be baked is pottery, a mullite-based material is preferable.
The auxiliary blanket 11 makes the periphery of the blanket 10 an insulating space and suppresses the occurrence of a temperature gradient due to the heat radiation from the blanket 10 to the surrounding atmosphere thereof. Therefore, the auxiliary blanket 11 is made of an insulating material such as alumina fiber or foamed alumina, which has insulating properties and permits microwaves to be transmitted therethrough, similar to the outer wall 25a of the blanket 10.
As described above, when the partition wall 25 of the blanket 10, which partitions the baking chamber 23, is comprised of the inner wall 25b capable of transmitting part of microwaves to the inside of the baking chamber 23 while self-heating by the microwave, and the outer wall 25a which is made of an insulating material and surrounds the inner wall, the atmosphere temperature inside the baking chamber 23 rises by the self-heating of the inner wall 25b and the heat radiation from the baking chamber 23 to the outside is suppressed by the outer wall 25a, simultaneously with the progress of the microwave heating to an object to be baked.
Therefore, the atmosphere inside the baking chamber 23 is kept stable at a high temperature according to the temperature rising of the object 21 to be baked so that the heat radiation from the surface of the object 21 to be baked to the periphery thereof can be suppressed.
As a result, a temperature gradient between the central portion of the object to be baked and the surface thereof hardly occurs, and crack is prevented from occurring due to the temperature gradient. Thus, the baking can be performed stably.
However, in the conventional partition wall 25, the outer wall 25a for the main purpose of insulation and the inner wall 25b for the main purpose of heating constitute a double wall structure in a state in which they are closely adhered to each other. Therefore, when the temperature of the inner wall 25b rises to a high-temperature zone at a time or the inner wall is cooled down after baking, a significant thermal shock acts between the outer wall 25a and the inner wall 25b due to the difference in thermal expansion therebetween. As a result, the inner wall 25b made of, for example, a mullite-based material may be easily broken, and the life span of the double wall structure for preventing the occurrence of the temperature gradient may be shortened.
Further, the mullite-based material used for the inner wall 25b shows high heating characteristics near the baking temperature of the object 21 to be baked, but shows low heating characteristics in a low-temperature zone including room temperature. Therefore, at the time of initial temperature rising in a low-temperature zone by the microwave heating, the self-heating value of the inner wall 25b is small. Thus, a problem remains unsolved that when an object to be baked whose dielectric loss is small at room temperature is baked, it is difficult to efficiently heat the object to be baked, similar to the conventional baking furnace.